The tungsten lamp is and has been the most common source of light for applications requiring a relatively intense light source such as projectors, optical lens systems and similar applications. Unfortunately, such structures are configured in a manner which tends to develop undesired heat and, in turn, necessitates expensive and cumbersome cooling devices located immediately adjacent the light source in order to provide the required cooling. Also, such structures tend to have an inherent problem in that the life of the light source is relatively short, about 10 to 20 hours of operational life, for example. Thus, it is a common practice to replace the light source of the structures each time the system is to be employed. Obviously, the inconvenience and expense of light source replacement each time the apparatus is used leaves much to be desired.
An improvement over the above-described tungsten lamp system is provided by a system utilizing a high intensity discharge lamp as a light source. For example, a common form of HID lamp is the high pressure metal halide discharge lamp as disclosed in U.S. Pat. No. 4,161,672. Therein is disclosed a double-ended arc tube configuration or an arc tube having electrodes sealed into diametrically opposite ends with an evacuated or gas-filled outer envelope. However, the manufacture of such double-ended structures is relatively expensive and the configuration is obviously not appropriate for use in projectors and similar optic-lens types of apparatus.
An even greater improvement in the provision a light source for projectors and optic-lens apparatus is set forth in the single-ended metal halide discharge lamps as set forth in U.S. Pat. Nos. 4,302,699; 4,308,483; 4,320,322; 4,321,501 and 4,321,504. All of the above-mentioned patents disclose structure and/or fill variations which are suitable to particular applications. However, any one or all of the above-mentioned embodiments leave something to be desired insofar as arc stability and minimal color separation capabilities are concerned.